[teaearlgraycold]

If we’re talking about human technology available in a few hundred years, don’t discount far more exotic options. I’ve heard people talk of theoretical terrestrial lasers pushing on tiny probes. With an absolutely gigantic laser and magical material at the back of the probe that won’t instantly vaporize there’s enough energy to get something the size of a smartphone up to a reasonable proportion of the speed of light.

I can’t prescribe this theoretical technology to the problem. But I also think it’s unreasonable to set the limit using known technology and then discount the idea altogether. We have no idea what will be possible in 300 years.

[mr_mitm]

Note that you can't use these lasers to slow down the probe, which will dramatically limit the things the probe can do at the destination. I'm not even sure what kind of interesting things a probe the size of a smartphone could do, let alone phone home.

[perilunar]

> you can't use these lasers to slow down the probe

You can, but you need a couple of solar sails. You beam the laser at the sails to accelerate, then at the halfway point one of the sails detaches and becomes a reflector, which lights up the now flipped probe and remaining sail to decelerate them.

[throwawayben]

send an unending chain of them and you solve the transmission problem _and_ they don't have to slow down as you'll always have another on the way past whatever you're targeting

[zelphirkalt]

How many times the Earth's materials would we consume doing that? And wouldn't it block ma lazor from reaching the previous probes and accelerating them?

[pandoro]

Exactly. Imagine what would be possible after a billion year of technological evolution, heck even just 100'000 years. We already know that space time metric engineering is theoretically possible within our current understanding of physics, we don't have either the technology or access to energy density necessary to do it. And that's only within our limited understanding of how the universe works.

[jl6]

If you have a billion year long civilization, it would probably be easier to genetically/medically engineer humans to live forever, so that a 75,000 year space voyage doesn’t seem so long. Easier than near-lightspeed space travel at any rate.

[jimbokun]

Could you expand on what “space time metric engineering” is?

[pandoro]

The general idea is to deliberately shape the geometry of spacetime to allow for effects such as faster-than-light travel (relative to another region of spacetime).

Einstein's field equations link geometry to matter and energy:

G_μν = 8πG/c⁴ · T_μν

Left side describes how spacetime curves (the metric tensor). Right side represents what's causing the curvature (the stress-energy tensor): mass, energy, momentum. The usual approach is: you specify T_μν (what matter is where) and solve for the resulting curvature. Metric engineering takes the backward approach. You pick the geometry you want first: e.g a bubble that carries its contents faster than light relative to the outside universe. Then you work out what T_μν would have to be to produce that geometry: what distribution of energy and stress the equations demand.

The classic example of spacetime metric engineering is the Alcubierre drive.

The limitation with this is that the required T_μν has negative energy density in some regions (the bubble walls) which violate some assumptions about how "reasonable" matter behaves. But Quantum Field theory allows such local regions of negative densities and we have created such regions in labs experimentally already (Casimir effect). We "just" don't know how to sustain the effect at bigger (atomic) distances.

[perilunar]

At the distances involved even the beam from a gigantic laser will be spread out enough not to vaporise anything — you won't need a magic material, just a very large, thin, and highly reflective solar sail.